Using extract-text-webpack-plugin would solve your problem. What it does is moving your generated content (in this case css) to a separate file (instead of bundling it together in your bundle.js file).

Install it via npm:

npm install --save-dev extract-text-webpack-plugin

Update your webpack config to include this:

var ExtractTextPlugin = require('extract-text-webpack-plugin');

Then replace your sass loaders with its customised loader, and include the plugin: